An optical isolator, or optical diode, is an optical component which allows the transmission of light in only one direction. It is typically used to prevent unwanted feedback into an optical oscillator, such as a laser cavity.
An optical isolator is a device that only allows unidirectional transmission of the optical signal. It is often used in optical systems to avoid unwanted optical reflections. For example, a single-frequency semiconductor laser is very sensitive to external optical feedback.
Types of Optical IsolatorOptoisolators are classified into three types which include Polarized, Composite, and Magnetic optical-isolator
Polarized Type Optical-IsolatorThis isolator uses the polarization axis to keep light transmit in one direction. It allows light to transmit in forwarding direction, however, prohibits every light beam to transmit back. Also, there are dependent and independent polarized optical-isolators. The latter is more complicated and often used in EDFA optical amplifier.
Composite Type Optical-IsolatorThis is an independent polarized type optical-isolator, which can be used in EDFA optical amplifier which includes different components like wavelength-division multiplexer (WDM), erbium-doped fiber, pumping diode laser, etc..
Magnetic Type Optical-IsolatorThis type of isolator is also named as the polarized optical-isolator in a new face. It pressures the magnetic element of a Faraday rotator, which is usually a rod designed with a magnetic crystal beneath the strong magnetic field through Faraday Effect.
Faraday Isolators
A Faraday isolator is a kind of optical isolator, i.e., a device which transmits light in a certain direction while blocking light in the opposite direction. Faraday isolators are based on Faraday rotators and constitute the technologically most important type of optical isolators.
A variant is an optical circulator, having at least three optical ports. Light injected into port 1 will exit at port 2, while input at port 2 will be sent to port 3, and input at port 3 to port 1.
Polarization-sensitive Faraday Isolators
The simplest type of Faraday isolator is polarization-sensitive in the sense that it works only when the input beam has a prescribed direction of linear polarization. Here, a properly polarized and collimated input beam passes a first polarizer (pol 1 in Figure 1) with minimum loss, then a 45° Faraday rotator, and finally another polarizer (pol 2) with its transmitting axis being rotated by 45°, such that the transmission losses are small.